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An expert witness or professional witness is a witness who can provide expert testimony within legal processes. By virtue of education, training, skill, or experience, they are believed to have knowledge in a particular subject beyond that of the average person, sufficient that others may officially (and legally) rely upon the witness's specialized (scientific, technical or other) opinion about an evidence or fact issue within the scope of their expertise, referred to as the expert opinion, as an assistance to the fact-finder.[1] Expert witnesses may also deliver expert evidence about facts from the domain of their expertise.[2] At times, their testimony may be rebutted with a learned treatise, sometimes to the detriment of their reputations.

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Typically, experts are relied on for opinions on severity of injury, degree of insanity, cause of failure in a machine or other device, loss of earnings, care costs, and the like. In an intellectual-property case, an expert may be shown two music scores, book texts, or circuit boards and asked to ascertain their degree of similarity.

The tribunal itself, or the judge, can in some systems call upon experts to technically evaluate a certain fact or action, in order to provide the court with a complete knowledge on the fact/action it is judging. The expertise has the legal value of an acquisition of data. The results of these experts are then compared to those by the experts of the parties.

The expert has a heavy responsibility, especially in penal trials, and perjury by an expert is a severely punished crime in most countries. The use of expert witnesses is sometimes criticized in the United States because in civil trials, they are often used by both sides to advocate differing positions, and it is left up to a jury of laymen to decide which expert witness to believe. Sometimes one side has utilized an expert witness to provide fraudulent or junk sciencetestimony in order to convince a jury. Such experts are commonly disparaged as "hired guns."

In England and Wales, under the Civil Procedure Rules 1998 (CPR), an expert witness is required to be independent and address his or her expert report to the Court. A witness may be jointly instructed by both sides if the parties agree to this, especially in cases where the liability is relatively small.

Under the CPR, expert witnesses are usually instructed to produce a joint statement detailing points of agreement and disagreement to assist the court or tribunal. The meeting is held quite independently of instructing lawyers, and often assists in resolution of a case, especially if the experts review and modify their opinions. When this happens, substantial trial costs can be saved when the parties to a dispute agree to a settlement. In most systems, the trial (or the procedure) can be suspended in order to allow the experts to study the case and produce their results. More frequently, meetings of experts occur before trial.

In the U.S., a party can hire experts to help him/her evaluate the case. For example, a car maker may hire an experienced mechanic to decide if its cars were built to specification. This kind of expert opinion will be protected from discovery. If the expert finds something that is against its client, the opposite party will not know it. This privilege is similar to the work product protected by the attorney/client privilege.

If the witness needs to testify in court, the privilege is no longer protected. The expert witness's identity and nearly all documents used to prepare the testimony will become discoverable. Usually an experienced lawyer will advise the expert not to take notes on documents because all of the notes will be available to the other party.

An expert testifying in court must satisfy the requirements of Fed. R. Evid. 702.[3] Generally, under Rule 702, an expert is a person with “scientific, technical, or other specialized knowledge" who can "assist the trier of fact,” which is typically a jury. A qualified expert may testify “in the form of an opinion or otherwise” so long as: “(1) the testimony is based upon sufficient facts or data, (2) the testimony is the product of reliable principles and methods, and (3) the witness has applied the principles and methods reliably to the facts of the case.”

The Federal Court of Australia has issued guidelines for experts appearing in Australia courts[4]. This covers the format of the expert's written testimony as well as their behaviour in court. Similar procedures apply in non-court forums, such as the Australian Human Rights and Equal Opportunity Commission[5].